Process for oxidizing polyalkylaromatic hydrocarbons and derivatives thereof using an acid anhydride catalyst



United States Patent TIVES THEREOF USING AN ACID ANHY- DRIDE CATALYST Robert H. Saunders, West Chester, Pa., assignor to Hercules Powder Company, Wilmington, DeL, a corporation of Delaware No Drawing. Application August 21, 1952, Serial No. 305,679

16 Claims. (CI. 260-475) PROCESS FOR This invention relates to the oxidation of polyalkylaromatic hydrocarbons to yield diificultly oxidizable aromatic carbon lic acids and to the oxidation of esters of said difiicultly oxidizable aromatic carboxylic acids to yield esters of aromatic polycarboxylic acids. It especially relates to the oxidation of the various xylene isomers to yield the corresponding toluic acids and to the oxidation of esters of said toluic acids to yield monoesters of the corresponding phthalic acids. Thus, for example, it relates to the oxidation of p-xylene to p-toluic acid and to the oxidation of esters of p-toluic acid to yield monoesters of terephthalic acid.

There is known a highly advantageous process for converting p xylene practically quantitatively into terephthalic acid or its esters at elevated temperatures solely with the aid of oxygen or oxygen-containing gases. In this process the terephthalic acid-containing p-toluic acid obtained as an intermediate product is esterified. The terephthalic acid ester, if desired, is removed, and the p-toluic acid ester is oxidized with oxygen or oxygencontaining gases. Surprisingly, it has been found that in the oxidation of p-toluic acid esters with oxygen practically only the nuclear methyl group is attacked while the alcohol radical combined with the carboxyl group of the p-toluic acid remains unattached.

it is known that the oxidation of p-toluic acid esters with oxygen or oxygen-containing gases at elevated temperatures in liquid phase, under pressure if desired, and advantageously in the presence of oxidation catalysts, proceeds at a rather high rate and gives excellent yields of pure terephthalic acid monoesters. Moreover, under conditions of oxidation practically no ester interchange of the terephthalic acid monoester occurs so that the latter is obtained in practically pure form.

it is also known that in the oxidation of polyalkylaromatic compounds other than the xylenes, the rate of oxidation to the corresponding aromatic polycarboxylic acids is in no way uniform, that also in the oxidation of the other polyalkylaromatic compounds intermediate carboxylic acids are formed which, like p-toluic acid in the case of p-xylene, are much harder to oxidize than the starting material or other intermediates, so that oxidation to polycarboxylic acids undergoes a marked retardation.

As in the oxidation of p-xylene, the rate of oxidation of other polyalkylaromatic compounds to the corresponding polycarboxylic acids also may be greatly increased if the intermediate carboxylic acids formed, which are relatively more difi-lcultly oxidizable than the starting material or other intermediates, are converted into more readily oxidizable esters, and these are further oxidized. Thus, difiicultly further oxidizable carboxylic acids occurring as intermediates, which, therefore, contain besides one or more carboxyl groups also alkyl radicals and/0r partially oxidized alkyl radicals, are esterified and the Ifisulting esters are further oxidized.

The aforesaid process can be applied not only to the various methyl benzenes, such as xylenes, mesitylene, pseudo-cumene, durene, etc., but also to polyalkylaromatic compounds with long alkyl chains such as methyl ethyl benzene, cyrnenes, etc., and also to alkylated polynuclear aromatic compounds such as naphthalene, diphenyl, anthracene, etc.

I have found in experimenting with the aforesaid process that for some unknown reason there are occasionally encountered induction periods, not only in the oxidation of the starting material; i. e., the polyalkylaromatic hydrocarbon, but also in the oxidation of the intermediate esters of the dificultly oxidizable aromatic carboxylic acids. Thus, one occasionally finds that in the oxidation or" p-xylene, for example, there is an induction period of from 12 to 24 hours or more in which no oxidation takes place. Similar induction periods obtain in the case of the oxidati n of, for example, the methyl ester of p-toluic acid.

I have also found that it is possible to reduce these induction periods considerably, and to practically eliminate them in some cases, by the simple expedient or" adding a small amount of an organic acid anhydride to the material to be oxidized. Hence the invention broadly expressed comprises adding a small amount of an organic acid anhydride to a polyalkylaroznatic hydrocarbon or an ester of a difficultly oxidizable aromatic carboxylic acid derived from ,a polyalkylaromatic hydrocarbon by oxidation thereof and then oxidizing said hydrocarbon or said ester in liquid phase with an oxygen-containing gas at an elevated temperature, preferably in the presence of an oxidation catalyst.

Any organic carboxylic acid anhydride may be used for the purpose of this invention. Thus, aliphatic carboxylic acid anhydrides such as acetic, propionic, butyric, valeric, etc., acid anhydrides may be used. Similarly, aromatic carboxylic acid anhydrides such as phthalic acid anhydride, etc., may be used. There are, of course, several factors to be considered in the choice of an acid anhydride for this purpose: the cost and the ease with which any excess can be removed from the reaction mixture. Acetic anhydride is particularly advantageous from these points of View. The excess acetic anhydride is converted to acetic acid during the oxidation and distills from the oxidation vessel.

The minimum amount of organic carboxylic acid anhydride which it is necessary to employ to efiect a substantial reduction of the induction periods is variable and is apparently dependent on the amount of inhibitors present in the material being oxidized. Stated generally, it can be said that at least enough organic carboxylic acid anhydride should be employed to effect a substantial reduction of the induction period. In practice, amounts as low as 0.2% by weight based on the material being oxidized have been sufiicient. Rarely is it ever necessary to add amounts of anhydride above 4%. Use of additional anhydrlde merely adds to the cost of the process without effecting additional significant reduction of the induction period. Ordinarfly, from 1% to 2% by weight based on the material being oxidized is suflficient.

The use of organic acid anhydrides as described herein is important since shortening the induction period of an oxidation reaction will provide greater productivity in a given piece of equipment. The invention, accordingly, makes for economy of operation.

Now, having indicated in a general way the nature and the purpose of this invention, the following examples are given to illustrate it. They are not, however, to be construed as limiting the invention. In the examples, any percentages are by weight unless otherwise indicated.

Example 1 500 grams of a commercial xylene having the following composition:

was blown with air at the rate of about 0.75 liter per minute at 120 C. in the presence of 0.5 gram of cobaltous caproate for 13 hours without any oxidation taking place. Another 500-gram charge of this xylene was blown with air under the same conditions, only grams of acetic anhydride was added to the charge at the start of the blowing. Within 45 minutes oxidation had started as evidenced by the evolution of water and the development of the characteristic green color of oxidation. At the end of 12 hours the oxidate contains 150 grams of toluic acids.

Example 2 A 2000-gram charge of mixed xylenes (a commercial product containing 66% rn-xylene, 33% p-xylene, and the remainder o-xylene and ethyl benzene) was air-oxidized at 125 C. under atmospheric pressure in the presence of the cobalt salt of Cs-Ciz fatty acids obtained in the oxidation of paraflin as catalyst for a period of hours. The catalyst was employed in the amount of 0.2% based on the xylene, the air rate was about 6 liters per minute. At the end of this 25 hour period, the acid number of the oxidate was 250. The oxidate was filtered at 125 C. to remove the crystalline phthalic acids that had been formed therein. The filter cake, after washing with xylene, weighed 100 grams and was found to be 70% phthalic acids and toluic acids.

The liquid portion of the oxidates; i. e., the filtrate, was subjected to distillation at 2225 mm. Hg absolute pressure to give a xylene cut of 1000 grams, a toluic acid cut of 1000 grams, and a residue of 260 grams. The toluic acid fraction was then esterified with methanol in two steps. The toluic acid and 708 grams of anhydrous methanol were heated in an autoclave at 230 C. and 600 p. s. i. (autogenous pressure) for a period of about two hours. The autoclave was then vented at 190 C. and again charged with 708 grams of anhydrous methanol and further heated at 230 C. and 600 p. s. i. for two hours. 1100 grams of methyl toluates having an acid number below 5 were obtained in this manner.

500 grams of the methyl toluates was blown with air at the rate of about 0.75 liter per minute at 160 C. in the presence of 0.5 gram of the cobalt salt of C6-C12 fatty acids, obtained in the oxidation of parafiin, for 24 hours without any oxidation taking place. Another 500- gram charge of the methyl toluate was then blown with air under the same conditions only 12 grams of acetic anhydride was added to the charge at the start of the blowing. By the time the reaction temperature of 160 C. had been reached (about 30 minutes) the characteristic green color of oxidation had developed in the charge indicating that oxidation was taking place, and 15 minutes later reaction water was observed in the trap. At the end of 21 hours the oxidate contained 165 grams of monomethyl phthalate.

The above examples illustrate the efiect of acetic anhydride in reducing the induction periods in the oxidation of mixed xylenes and mixed methyl toluates. It has essentially the same efiect when used in the oxidation of the commercially available xylene isomers, p-xylene, mxylene, and o-xylene, and when used in the oxidation of the corresponding methyl toluates.

What I claim and desire to protect by Letters Patent is:

1. In the process which comprises oxidizing a material selected from the group consisting of a polyalkylaromatic hydrocarbon and an ester of a carboxylic acid derived from said polyalkylaromatic hydrocarbon by oxidation thereof and containing at least one substituent selected from the group consisting of alkyl and partially oxidized alkyl groups, in liquid phase with an oxygencontaining gas at a temperature of from about C. to about 250 C. in the presence of an oxidation catalyst to produce a carboxylic acid, the step of adding a small amount of an organic acid anhydride to said material to overcome an induction period.

2. In the process which comprises oxidizing a material selected from the group consisting of a xylene and an ester of a toluic acid in liquid phase with an oxygencontaining gas at a temperature of about 80 C. to about 250 C. in the presence of an oxidation catalyst to produce a carboxylic acid, the step of adding a small amount of an organic acid anhydride to said material to overcome an induction period.

3. The process of claim 1 wherein the oxidation is carried out in the presence of a cobalt salt as oxidation catalyst.

4. The process of claim 3 in which acetic anhydride is added to the reaction mixture to overcome an induction period.

5. The process of claim 4 wherein a polyalkylaromatic hydrocarbon is oxidized.

6. The process of claim 4 wherein an ester of a carboxylic acid derived from a polyalkylaromatic hydro carbon by oxidation thereof and containing at least one substituent selected from the group consisting of alkyl and partially oxidized alkyl groups is oxidized.

7. The process of claim 6 wherein the methyl ester is oxidized.

8. The process of claim 2 wherein the oxidation is carried out in the presence of a cobalt salt as oxidation catalyst.

9. The process of claim 8 wherein acetic anhydride is added to the reaction mixture to overcome an induction period.

10. The process of claim 9 wherein p-xylene is oxidized.

11. The process of claim 9 wherein m-xylene is oxidized.

12. The process of claim 9 wherein o-xyleneis oxidized.

13. The process of claim 9 wherein a methyl ester of a toluic acid is oxidized.

14. The process of claim 13 wherein methyl p-toluate is oxidized.

15. The process of claim 13 wherein methyl m-toluate is oxidized.

16. The process of claim 13 wherein methyl o-toluate is oxidized.

References Cited in the file of this patent UNITED STATES PATENTS 2,245,528 Loder June 10, 1941 2,302,462 Palmer et al. Nov. 17, 1942 2,458,207 Randolph Jan. 4, 1949 2,479,067 Gresham Aug. 16, 1949 2,653,165 Levine Sept. 22, 1953 FOREIGN PATENTS 623,836 Great Britain May 24, 1949 

1. IN THE PROCESS WHICH COMPRISES OXIDIZING A MATERIAL SELECTED FROM THE GROUP CONSISTING OF A POLYALKYLAROMATIC HYDROCARBON AND AN ESTER OF A CARBOXYLIC ACID DERIVED FROM SAID POLYALKYLAROMATIC HYDROCARBON BY OXIDATION THEREOF AND CONTAINING AT LEAST ONE SUBSTITUENT SELECTED FROM THE GROUP CONSISTING OF ALKYL AND PARTIALLY OXIDIZED ALKYL GROUPS, IN LIQUID PHASE WITH AN OXYGENCONTAINING GAS AT A TEMPERATURE OF FROM ABOUT 80*C. TO ABOUT 250*C. IN THE PRESENCE OF AN OXIDATION CATALYST TO PRODUCE A CARBOXYLIC ACID, THE STEP OF ADDING A SMALL AMOUNT OF AN ORGANIC ACID ANHYDRIDE TO SAID MATERIAL TO OVERCOME AN INDUCTION PEROID. 